[Patch, microblaze]: Add config/microblaze.mt for target_makefile_frag

Mirror MIPS method of creating copy of default.mt which drops the
compilation of generic sbrk.c to instead continue using the microblaze
provided version.

[Libgloss]

Changelog

2013-07-15  David Holsgrove <david.holsgrove@xilinx.com>

  * config/microblaze.mt: New file.
  * microblaze/configure.in: Switch default.mt to microblaze.mt.
  * microblaze/configure: Likewise.

Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>